1. Field of the Invention
The invention relates to a semiconductor laser source and, in particular, to a structure of elementary modules assembled by mechanical clamping.
2. Discussion of the Background
Since the 1970s, it has been proposed to exploit the advantageous properties of laser diodes (ease of fast direct modulation, high efficiency in the conversion of electrical energy into light energy in a narrow spectral band etc.) to produce devices capable of generating high power densities per unit area. To this end, monolithic elementary laser diodes were assembled by soldering in the form of stacks, as schematized in FIG. 1 in the case of four elementary diodes. It can be seen that, all other things being equal, the emitted power density is inversely proportional to the height H of the elementary diode.
Devices of this type were marketed in the 1970's by the company Laser Diode Laboratories, with a number of variants, differing by the number of stacked elements and the number of assembled stacks. The elementary laser diodes were single-heterojunction diodes which had a threshold current density of the order of 10.sup.4 A/cm.sup.2, which entailed operation in short pulses (typically 100 ns) with low recurrence frequencies (typically 1 kHz). Assemblies of this type could deliver peak powers of several kW; one intended application, among others, was the pulsed illumination of scenes in the near infrared.
The development of quantum-well laser diodes in the 1980s made it possible to improve considerably their properties, threshold current and differential efficiency, and consequently to increase the energy conversion efficiency, which may be up to 50% in long pulse operation (several hundreds of .mu.s) or in continuous operation.
These new characteristics have stimulated the application of laser diodes to the pumping of solid-state lasers, in particular YAG:Neodymium lasers, by replacing flash or other lamps, with an increase in the "take-up" efficiency of these lasers by a factor of more than 10. This increase is essentially due to the small spectral width (3 nm) of the laser diodes, compared with that of "white" sources. The typical operating mode, referred to as "quasi-continuous" or QCW, consists of "long" pulses (a few hundred .mu.s), and it is also beneficial to increase the recurrence frequency beyond the 100 Hz obtained with flash lamps. Products are marketed, for example by the company Spectra Laser Diodes, in the form of hybrid stacks as described in the document J. G. Endriz et al., High power diode laser arrays, IEEE J. Quantum Electron., 28(4), 952-965, April 1992, the design of which has the clear aim of allowing operating at High frequency, and consequently at high mean power. To this end, monolithic diodes in arrays, typically of 1 cm width, are soldered onto supports made of a material with high thermal conductivity, in the form of elementary modules which are themselves assembled by soldering on a common support, in a number depending on the power to be obtained, and are electrically connected in series.
This type of assembly has two drawbacks:
the cost of the assembly is high; PA1 the emitted power density is geometrically limited by the height of an elementary module.
The object of the invention is firstly to overcome the drawback of the cost for certain applications, pumping solid-state lasers, which require large numbers of stacks and will consequently be economically viable only with a substantial reduction in the fabrication costs, but do not require pulsed operation whose recurrence rate is high. The invention furthermore has the advantage of allowing an increase in the power density emitted.
The cost can be analyzed into two components.
1. The "front end", all the collective technologies (material epitaxy, microlithography, electrical contact metallization), has a cost per elementary device which decreases very rapidly as the quantities produced increase, as found throughout the history of silicon technology. This will inevitably be the case for diode laser arrays.
2. The cost of the "back end", all the assembly and encapsulation technologies, therefore becomes increasingly dominant as the functions fulfilled become more complex. This is the case for the stacks marketed today.